Switching Design Considerations

[Ian Moore]

Hi, I would like to know community members opinion on the following: Does the core/distribution/access & collapsed core switching models still feature in today's network designs or has the leaf & spine architectural model replaced this completely? I appreciate leaf and spine with top of rack switches is preferential for datacentre networks but what about campus networks where multiple access switch stacks are used for connecting end user devices? In this scenario East/West traffic is reduced significantly in comparison with datacentre networking. Would a traditional core/distribution/access or collapsed core network be considered a better design in this instance with each access switch stack connected to a modular or stacked core, or is leaf & spine with each access switch stack (leaf) connected to mulitple spine switches considered the better design?

[Seb Rupik · Accepted solution]

Hi there,

In this comparison I would focus on the Layer1 requirements the two different designs have. In a DC where infrastructure is in close proximity to each other and structured cabling/ fibre is plentiful, spline and leaf fits beautifully.

A campus network is a different beast. Port density requirements for access layer switches are extremely varied. Even supposing there were ACI switches which supported less that 48 ports, all of these small switches will require links back to the spine switches. A single building could quickly consume all of the ports on a set spine switches.

But suppose your spine switches did have enough capacity you still require a huge amount of fibre to connect these leaf switches spread around the building/campus directly to the spine switches. That is a lot of ducts!!

This is why the 3-tier network fits the campus so well. By having aggregation in the buildings and then connecting these to the core greatly reduces the amount of physical fibre.

Granted, if you had a heap of cash and were determined on implementing ACI on campus you could implement ACI Multi-Pod, each building would have a separate set of spine switches to serve just the leaf switches in that building. These separate pods can then communicate between themselves. However you are currently limited to 12 pods, so even then it may not be possible to fit on a large campus!

https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-737855.html

cheers,

Seb.

[Ian Moore]

Hi Seb, thanks for the info! I hadn't considered the scalability of the leaf & spine model so I can now see why the 3-tier model is a better fit for the campus network.

[Mark Malone · Accepted solution]

Hi
we still use access/dist/core in campus side , 4500s on access layer in VSS to a dual VSS dist setup with VSS 6509E cores , ISE policy driven access layer to cut down on manual config mgmt , all switches in phases of migrating to 9k campus now too , DCs TOR with ACI alright , dont think either is a better design but both have there benefits ,haven't seen many campus setups yet using ACI but plenty of DCs have taken it since its become more stable than the earlier releases, SD-LAN will eventually be common its already on the way with Cisco DNA being pushed , all depends on the size of the network and the requirements what you use , a collapsed core may be fine if small sized office too but you should try and segment L2 and L3 off the same switch when possible better security , ACI is great when it a massive spread network as make mgmt easier and centrally controlled

[Ian Moore]

Hi, thanks for the reply Mark.  It's great to have your opinion on design considerations and the info provided is really useful.

[Mark Malone]

Glad it was useful for you